Sensor assembly for strapped-down attitude and heading reference system

ABSTRACT

A low-cost two-axis rate and acceleration sensor utilizing piezoelectric generator elements affixed to the rotating housing of an inside-out synchronous motor. Signals generated by the piezoelectric bender elements are amplifier, compensated, balanced, and converted to FM signals for transmission off the rotating assembly.

BACKGROUND OF THE INVENTION

This invention relates generally to reference apparatus for navigablevehicles such as aircraft, and more particularly, to rate/accelerationsensors used in an attitude and heading reference system. Advancement inthe art of precision flight control and guidance apparatus for aircraft,missiles, and space vehicles depends in part on progress in sensortechnology. Present computer technology allows sophisticated and complexsignal processing at reasonable cost, but the information processed isfrequently derived from sensors having a cost which is adisproportionate part of the system cost.

A reference system having inertial instruments rigidly fixed along avehicle-based orientation reference wherein the instruments aresubjected to vehicle rotations and the instrument outputs are stabilizedcomputationally instead of mechanically is termed a gimballess orstrapped-down system. Such systems generally include computing meansreceiving navaid data such as magnetic and radio heading; air data suchas barometric pressure, density, and air speed; along with outputsignals of the inertial instruments for generating signalsrepresentative of vehicle position and orientation relative to a systemof coordinate axes, usually earth oriented. The presence of high angularrates associated with strapped-down systems adversely affectsperformance and mechanization requirements. Consequently, such referencesystems have been used extensively in missiles, space, and militaryvehicles, but their use in commercial aircraft has been less extensivebecause of economic constraints associated with the manufacture ofprecision mechanical assemblies, i.e., gyroscopes and other precisionsensors. Strapped-down inertial reference systems become practical forcommercial aircraft from the standpoint of cost of ownership, weight,reliability, and maintainability with the advent of small, lightweight,highly accurate and relatively low-cost rate sensors and accelerometers.Angular rate sensing apparatus utilizing rotating piezoelectricgenerators are known; see for example U.S. Pat. Nos. 2,716,893 and4,197,737. Such devices generally comprise piezoelectric generatorelements mounted to a rotatable drive shaft and oriented for generatingsignals responsive to particular bending forces sensed by theinstrument; the processing of signals derived from such instrumentationinvolves the measurement, amplification and transmission of very lowlevel DC and low frequency signals. Prior art devices have exhibitedsignal degradations which make the devices unsuitable for someapplications. For example, the signals being processed may containundesirable carrier harmonics, DC bias, and other noise components, suchas those caused by signal phase shifts and mechanical misalignments inthe system, which undesirable components must be rejected to preventdegradation of the low-level signals of primary interest.

In view of the problems of present state-of-the-art sensors, describedabove, it is a general objective of the present invention to develop animproved low-cost sensor for generating signals representative ofvehicle accelerations and angular rates.

A more specific object of the invention is to provide an improvedmulti-function sensor for measuring angular rate about two axes andlinear acceleration along two axes, having relatively simple mechanicalconstruction, low bias drift, and high sensitivity.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, there is provided animproved, low cost and compact sensor assembly which utilizes aninside-out hysteresis motor having piezoelectric generator benderelements mounted exteriorly on a rotating housing of the motor. Circuitboards mounted on the rotating motor housing carry circuits whichamplify and convert the low-level DC signals generated by the benderelements for coupling off the rotating element via air-coretransformers.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is pointed out with particularity in the appended claims;however, specific objects, features, and advantages of the inventionwill become more apparent and the invention will best be understood byreferring to the following description of the preferred embodiment inconjunction with the accompanying drawings, in which:

FIG. 1 is a pictorial view, partially cut away, of a sensor assembly inaccordance with the present invention.

FIG. 2 is a section view of a sensor assembly in accordance with theinstant invention.

FIG. 3 is a pictorial view of a piezoelectric bender element utilized inthe practice of the invention.

FIG. 4 is a schematic block diagram of the sensor assembly includingcircuit elements, both on and off the rotating structure, associatedtherewith.

FIG. 5 is a diagramatic representation of linear acceleration sensorsuseful in explaining the operation of the present invention.

FIGS. 6A and 6B are diagramatic representations of angular rate andlinear acceleration sensors useful in explaining the operation of thepresent invention.

FIG. 6C is a vector diagram useful in explaining the operation of thepresent invention.

FIG. 6D is a simplified block diagram of one embodiment of compensationmeans utilized in the present invention.

FIG. 7 is a detailed electrical schematic diagram of the sensor assemblyof the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the various views of the drawing for a more detaileddescription of the components, materials, construction, operation andother features of the invention by characters of reference, FIGS. 1 and2 show a sensor assembly 10 constructed in accordance with the presentinvention. The sensor assembly 10 comprises a hysteresis motor 12 havinga fixed axial shaft 14. The shaft 14 is mounted and rigidly affixed, ateither end thereof, to a structural member 16, 16¹ having an orientationfixed with respect to a vehicle (not shown) in which the sensor assembly10 is installed. A rotating element of the motor 12 comprises acylindrical motor housing or sleeve 18 journaled for rotation about theshaft 14 on bearings 20, 22, the spin axis 23 of the sleeve 18 beingcoaxial with the shaft 14. The bearings 20, 22 are end-cap rollerbearings bonded with a preload to the shaft 14 and pressed into the endsof the motor housing 18. A stator 24 and its associated windings 26surrounds the shaft 14 and is affixed thereto. Leads 28 for supplyingalternating current to the stator windings 26 emanate from the motor viaa central bore 30 in the shaft 14. A cylindroid hysteresis ring 32 ofpermanent-magnet material is mounted interiorly of the motor housing 18for rotation therewith between a pair of ring spacers 34, 34¹. Thespacers, 34, 34¹ are made from a non-magnetic material such asaustenitic stainless steel, the preferred material being 304 stainlesssteel. The hysteresis ring 32 is juxtaposed with the stator 24, drivingthe rotating element in response to alternating current applied to theleads 28 of the stator windings 26. The motor housing 18 is made formartensitic stainless steel such as 416 stainless steel. The materialswere chosen to keep the bearing thermal expansion loop matched withrespect to coefficient of expansion so as to maintain the bearingpreload over temperature extremes. A motor such as the motor 12 having afixed shaft and stator, and an externally disposed rotating structure,is termed an inside-out motor.

A pair of piezoelectric (PE) crystal assemblies 36, 38 are mountedexteriorly on the rotating motor housing 18 on opposite sides thereof.Each of the crystal assemblies 36, 38 comprises a base 40 to which apiezoelectric bender element 42 is mounted in cantilever fashion. Acover 44 in cooperation with the base 40 extended encloses the benderelement 42. Leads 46 conduct output signals generated by thepiezoelectric bender elements 42 via feedthrough terminals 48 toelectronic circuits carried on the rotating element of the sensorassembly 10. The piezoelectric bender element 42 of the presentlydescribed embodiment is the same as described in my co-pending U.S.patent application Ser. No. 276,112, entitled Piezoelectric Sensor,assigned to the same assignee as the present invention, whichapplication is incorporated herein by reference.

Referring still to FIG. 1, a second pair of piezelectric crystalassemblies 50, 52 are shown mounted exteriorly on the rotating motorhousing 18 for measuring accelerations in the plane perpendicular to thespin axis 23 of the sensor assembly 10. The crystal assemblies 50, 52are mounted such that bending axes thereof are essentially parallel withthe spin axis 23 of the sensor assembly 10; axis of sensitivity,perpendicular to the spin axis 23. While the crystal assemblies 50, 52,are shown mounted orthogonally of each other, they can be mounted withtheir axes of sensitivity displaced with respect to each other by anyangle, or one could be omitted; however, a nominal 90° separationbetween two crystal assemblies is the preferred arrangement for reasonswhich will be explained hereinafter.

A pair of annular circuit boards 54, 56 are mounted exteriorly of themotor housing 18 for rotation therewith by any suitable means such ascollars 58, 58¹. A cylindrical plate 59 (see FIG. 2) extending betweenthe peripheral ends of the circuit boards 54, 56 encloses the spacetherebetween, serving as a dust cover for the circuit devices carried onthe boards. The enclosure formed by the circuit boards 54, 56 and theplate 59 protects the PE crystal assemblies 36, 38, 50, 52 fromturbulent air currents which could be generated if the rotatingcomponents were not so protected. Referring to FIG. 2, an annular,air-core power transformer having a stationary primary coil 60 affixedto the shaft 14, and a rotating secondary coil 62 concentric with theprimary coil 60 and mounted inside the rotating sleeve 18 supplies powerto electronic circuits on the circuit boards 54, 56. Power leads 64transmitting alternating current are routed through the central bore 30of the shaft 14 and connected to the primary coil 60 of the powertransformer. Similarly, power leads 66 from the secondary coil 62 of thetransformer are routed via a slot 67 in the motor housing 18 to thecircuit boards 54, 56. An annular, air-core signal transformer having aprimary coil 68 mounted inside the sleeve 18 for rotation therewith anda secondary coil 70 affixed to the shaft 14 couples output signals ofcircuits (components of which are shown in FIG. 1) on circuit boards 54,56 via leads 72 routed though a slot 73 in the sleeve 18 to the primarycoil 68, and from the secondary coil 70 via leads 74 routed through acentral bore 75 of the shaft 14 to user circuits external of the sensorassembly 10.

Referring now to FIG. 3, there is shown in greater detail apiezoelectric bender element 78 like the bender elements 42 of FIG. 2.The bender element 78 comprises a cantilevered piezoelectric-crystalsensor 80 affixed to a mounting member or base 82. The bender element 78generates a voltage V_(o) on output leads 83, 84 which is proportionalto the bending moment generated by accelerations acting on the mass ofthe sensor 80 itself along an axis of sensitivity illustrated by theline 85, the bending axis of the bender element 78 being in the plane ofthe sensor 80 and parallel with the longitudinal dimension of the base82. The bender element 78 is inherently sensitive without the additionof proof mass. Additional details relating to construction and operationof the bender element 78 may be found in the aforementioned co-pendingapplication Ser. No. 276,112.

Referring to FIG. 1, the desired accelerations are available when thepiezoelectric bender elements 36, 38, 50, 52 are rotated at a fixedfrequency, 3120 revolutions per minute in the presently describedembodiment, about the spin axis 23 as shown in FIG. 1.

A measurement of linear acceleration is accomplished simply be measuringthe phase and amplitude of the AC signal obtained by rotating the benderelements 50, 52 in the plane of interest. For rate measurement, thepiezoelectric bender elements 36, 38 are oriented as shown in FIGS. 1and 2 to sense acceleration along the spin axis 23. The bender elements36, 38 are rotated at the fixed spin frequency, N, in radians persecond, the spin axis 23 being oriented and fixed, for example, alongthe roll axis of an aircraft, and the sensor assembly 10 measures pitchrate and yaw rate. As the aircraft experiences an angular rateperpendicular to the spin axis, a useful coriolis acceleration along thespin axis acts on the mass of the bender elements 36, 38. At a point (r,α) on the bender element, the acceleration along the spin axis is givenby

    a=2Nrθ cos (Nt-α)+2Nrψ sin (Nt-α)    (1)

where θ and ψ are pitch and yaw rates, r is the radial distance from thespin axis to the point on the bender element and α is the angularlocation of the point on the bender element from the axis about whichangular rate is being sensed. A sinusoidal output voltage V_(o) isgenerated by the bender element as a result of the force and stressexerted on the piezoceramic material therein. The output voltage is ofthe form:

    V.sub.o =KθN cos Nt+KψN sin Nt                   (2)

where K is a sensitivity constant for the bender element, θ representsthe angular displacement or tilt of the spin axis in the Y-Z plane, andψ represents the angular displacement or tilt of the spin axis in theX-Z plane. θ and ψ are, respectively, dθ/dt and dψ/dt or angular rates,respectively, about the Y-axis and the X-axis as shown in FIG. 1. In theutilizing circuits, the output voltage V_(o) is phase detected andmeasured to determine the desired rates θ and ψ. V_(o) is an inherentlybias-free AC signal; signal-to-noise ratios of several dB are exhibitedat input rates equivalent to earth rate or less.

FIG. 5 illustrates an elementary form of a two-axis linear accelerometersuch as the PE crystal assemblies 50, 52 of FIG. 1. Anacceleration-sensitive device 90 such as a piezoelectric bender elementhaving an axis of sensitivity along a line 91 is affixed to a rotatingelement or shaft 92 rotating at an angular velocity ω_(n) about a spinaxis 93. Acceleration components anywhere in the plane perpendicular tothe spin axis 92 (the plane of the paper in FIG. 5) are measured; sincesuch a plane is defined by two axes, the illustrated sensor is referredto as a two-axes accelerometer. Assuming that FIG. 5 shows theacceleration-sensitive element 90 at a reference time ω_(n) t=0; that aforce on the positive side (+) of the element 90 generates a positivevoltage proportional to the force; and that the force is localacceleration a(t) acting on the mass of the acceleration-sensitivedevice 90, the because of the rotation, the voltage generated is of theform:

    v.sub.1 =Ka(t) sin (ω.sub.n t+α)               (3)

where K is a scale factor, a(t) is the local acceleration, α is an anglebetween a reference axis 94 of the acceleration-sensitive device 90 andthe acceleration force a(t), the latter represented by a line 95. Autilizing means would typically measure the amplitude and phase of thevoltage generated by the acceleration-sensitive device 90 to determinethe amplitude and direction of the acceleration a(t); however, a problemarises when the measured acceleration contains a component at afrequency 2ω_(n) as well as the normally steady-state or low frequencyacceleration (g) which is the quantity of interest to be measured. Thatis, if

    a(t)=g+a.sub.1 sin 2ω.sub.n t                        (4)

the voltage generated with reference to equation (3) contains a termwhich renders g indistinguishable from the 2ω_(n) accelerationcomponent. The problem is pervasive since rotating elements such as therotating element 92 characteristically comprise ball bearings whichcommonly generate a 2ω_(n) acceleration component.

The scale factor of the acceleration sensor is not determined by thespin speed of the sensor assembly since no coriolis term is involved,the coriolis term being perpendicular to the sensitive axis of theacceleration sensor. Assuming that a constant acceleration (such asgravity) component exists along the line 91, the acceleration sensor 90is bent by its own weight, and the bending direction is the samedirection as the shaft 92 rotation. After the shaft 92 rotates 180°, thebending moment is essentially equal and opposite; therefore, asinusoidal output signal is generated, the amplitude and phase dependingon the relative direction of the g field in the sensitive plane. Theaccelerometer sensitivity has the form:

    .sup.Q|.sub.a =d.sub.31 PW(x.sub.2 -x.sub.1).sup.3 a.sub.xy (t) cos (nt-α).                                         (5)

Note that spin speed n is not a scale factor, and that (x₂ -x₁) is thelength of the active sensor. If the acceleration a_(xy) (t) is asinusoidal vibration ω_(a), the charge output has a sinusoidal componentat frequencies (ω_(a) +N) and (ω_(a) -N). For most frequencies, theseterms do not result in steady-state outputs; however, certain harmonicfrequencies such as ω_(a) =2n cause potentially serious performanceproblems. At ω_(a) =2n a 1n signal and a 3n signal are generated. The 3nsignal is rejected, but the 1n signal has the appearance of steady-stateacceleration.

In order to measure the nominally steady-state component g in thepresence of a 2ω_(n) term, a second acceleration-sensitive device 96 ismounted to the rotating structure 92 and oriented at 90° from the firstdevice 90 as shown in FIG. 5. The devices 90, 96 may be angularlydisplaced from each other by any non-zero angle, however, a 90° offsetis preferred. If the sensitivity of the device 90, 96 are virtuallyidentical and the devices 90, 96 are physically oriented 90° apart, the2ω_(n) component will be cancelled if the output signal from the secondacceleration-sensitive device 96 is shifted in phase by 90° and added tothe output signal from the first device. Assume that a local gravityfield g exerts a force along the null axis 94 of the sensor 90 at ωt=0,and that a positive force on the + side of the sensors 90, 96 generates,respectively, positive voltages v₁ and v₂. Because the rotating element92 rotates at an angular velocity ω.sub. n, the sensor assembly acts asa modulator, and

    v.sub.out =Ka(t) sin ω.sub.n t                       (6)

Let

    a(t)=g+H sin (2ω.sub.n t+β)                     (7)

Then

    v.sub.1 =[g sin ω.sub.n t+H sin (2ω.sub.n t+β) sin ω.sub.n t]K.sub.1                                   (8)

    v.sub.2 =[g cos ω.sub.n t+H sin (2ω.sub.n t+β) cos ω.sub.n t]K.sub.2                                   (9)

Using the identities ##EQU1## Then ##EQU2## With only one sensor 90 or96, H corrupts the apparent amplitude and phase of the accelerationvector g, and a 3ω_(n) signal is generated and must be rejected.Ignoring in this instance the 3ω_(n) component, cancellation of the Hterms is accomplished if v₂ is delayed 90° electrically, whereby v₂delayed becomes v₂₂ and ##EQU3## adding (14) and (15) cancels the Hterms if the scale factors K₁ and K₂₂ associated with the sensors 90, 96are identical.

Referring again to FIG. 1, undesirable accelerations occurring in thespin plane (X,Y) of the sensor assembly 10 which may be caused bymechanical misalignments in the sensor assembly 10 and/or externallyapplied vibrations, apply force along the length of the rate sensors 36,38. If the electrical null axis of the sensors 36, 38 is exactly in theX,Y plane, no output signal resulting from the unwanted accelerations isgenerated; however, such exactness is achieved only through precisionmechanical assembly which precludes low-cost implementation. The presentinvention achieves low-cost implementation by providing electronic meansfor compensating for imprecise mechanical construction. Referring now toFIG. 6A, there is shown a simplified diagram of a pair of piezoelectricbender elements 100, 102 mounted on a rotating member 104 having a spinaxis along a line 105. The spin axis 105 is parallel with coriolisacceleration components to be measured by the bender elements 100, 102,the coriolis accelerations being developed when the spin axis 105 isrotated in space. The resulting acceleration is proportional to the rateof rotation of the rotating member 104 and is a well-known phenomenon.Referring now to the bender element 100 of FIG. 6A (the description,however, being applicable to either element 100, 102), the benderelement 100 is mounted on the rotating member 104 such that anacceleration-sensitive axis 106 of the sensor 100 is essentiallyparallel with the spin axis 105, i.e., as parallel as mechanicalconstruction will allow, but shown considerably offset in the drawingfor illustrative purposes. A null axis 107 of the sensor 100 exists suchthat steady-state or low frequency (relative to the resonant frequencyof the sensor 100) accelerations along the null axis 107 generate nooutput signals from the sensor 100. On the other hand, accelerationcomponents along the acceleration-sensitive axis 106 result in thegeneration of an electrical signal V_(c) in the sensor 100 which is ofthe form:

    V.sub.c =Kφω.sub.n sin (ω.sub.n t+α) (16)

When the sensor 100 is misaligned by an angle δ₁, from a true null axis107¹ of the assembly as shown in FIG. 6A and/or by an angle δ₂ from atrue null axis 107¹¹ as shown in FIG. 6B, and if a cross-axisacceleration 108 exists, then the sensor 100 generates a signal V_(ca)responsive to the cross-axis acceleration 108 which is of the form:

    V.sub.ca =Kaδ sin (ω.sub.n t+α)          (17)

where α is a phase shift dependent on the orientation of themisalignment. Under certain circumstances, the output resulting from themisalignments δ₁ and δ₂ are indistinguishable from the desired signal.Misalignments δ₁ and δ₂ can be eliminated or reduced to tolerable levelsby precise mechanical construction; however such precision constructionis costly. Referring now to FIGS. 6A-C, FIG. 6C is a simplified vectordiagram representative of the output signals developed by a misalignedacceleration-sensitive component such as the sensor 100. The existenceof misalignments δ₁ and δ₂ results in an interfering signal representedby the vector 110. An interfering signal such as the signal 110 can beexpected to be of random phase for non-precision assembly, i.e. thesignal 110 may fall in any quadrant of the FIG. 6C diagram. Means musttherefore be provided which compensate for an interfering signal at anyangle. Cross-axis acceleration compensation is accomplished inaccordance with the present invention by two additional accelerometers112, 114 mounted on the rotating member 104. The accelerometers 112, 114measure the accelerations in the entire plane 116 perpendicular to thespin axis 105. While the accelerometers 112, 114 are shown angularlydisplaced from each other by 90°, they can be mounted at any non-zeroangle with respect to each other; however, a nominal 90° separation isthe preferred alignment. Referring still to FIGS. 6A-C, it is assumedthat the accelerometers 112, 114 are aligned such that they nominallygenerate signals represented by vectors 118 and 119. Opposing vectors120 and 121 may be generated by inverting, respectively, signalsrepresentative of the vectors 118 and 119; since the offending vector110 can fall in any quadrant, a cancelling vector 110¹ must beconfigurable for any quadrant. A cancelling vector can be generated inany quadrant by effecting the sums of selected ones of the four vectors118, 119, 120, 121 of sufficient amplitude to form the desired vector.In the example illustrated in FIG. 6C, the cancelling vector 110¹ isgenerated by selecting proper amplitudes of the 3π/2 vector 119 and theπ vector 120. Referring now to FIG. 6D, the acceleration-sensitivedevices 100, 102, 112, 114 described with reference to FIGS. 6A and 6B,are represented in FIG. 6D as sine wave generators 100, 102, 112, 114.The output signals of the rate sensors 100, 102 are combined afteramplification in amplifiers having respective gains K5 and K6 in asumming means 122, the output signal of the summing means 122 comprisinga desired signal 123 representative of the sensed angular rates plus theinterfering signal 110. The output signals of the acceleration sensors112, 114 are utilized to form the cancelling vector 110¹ by adjustingthe appropriate gains K1, K2 corresponding respectively with thepositive and negative output signal vectors 118, 120 of accelerationsensor 112, and gains K3, K4 corresponding respectively with thepositive and negative output signal vectors 119, 121 of accelerationsensor 114, and selectively applying these signals to a summing means124 along with the summed output signals of the angular rate sensors100, 102. In the summing means 124, the cancelling vector 110¹compensates for the interfering vector 110, and the output signal 123consequently comprises only the desired angular rate components.

FIG. 4 is a simplified block diagram of a two-axis rate and accelerationsensor assembly such as the sensor assembly 10 of FIG. 1, and thecircuits associated therewith. The sensor assembly comprises a spinmotor having a fixed stator assembly represented by the block 140 and arotating assembly 141; circuits carried on the rotating assembly 141 aremounted on annular printed circuit boards previously described withreference to FIG. 1. The spin motor is a hysteresis synchronous motordriven by an inverter 142 operating from a regulated DC power source143. A power inverter 144 serves as an AC power surface for the circuitson the rotating assembly 141, the AC being coupled via an air-coretransformer 145 to a power supply on the rotating assembly 141. Thepower supply 146 rectifies and filters the AC and supplies DC operatingvoltage to the circuits carried on the rotating assembly 141. Timingcircuits 148 generate control signals and timing pulses forsynchronizing and controlling the operation of the sensor circuits. Thetiming circuits 148 receive an input derived from a precision clocksource, such as a 640 KHz crystal oscillator 149 shown in the presentlydescribed embodiment. A synchronizing signal representative of theposition of the rotating assembly 141 with respect to the fixed elementsof sensor assembly is coupled from a transducer 150 via a sync pulsedetector 151 to the timing circuits 148. The source of the synchronizingsignal may be a magnetic element 152 affixed to the rotating assembly towhich the transducer 150, e.g. a variable-reluctance coil, is responsiveas the magnetic element 152 passes the fixed element 150. The positionreference of the rotating assembly 141 may be generated alternatively byany suitable means such as optoelectronic devices.

Four miniature piezoelectric bender elements 154, 155, 156, 157 mountedon the rotating assembly 141 are used to sense the accelerations ofinterest. Two sensors 154, 155 are oriented with their sensitive axesparallel to the spin axis (as previously shown with reference to FIG. 1)to measure coriolis acceleration proportional to rates of turn aboutselected axes perpendicular to the spin axis. Two other sensors 156, 157are mounted with their sensitive axes perpendicular to the spin axis formeasuring linear accelerations in the plane perpendicular to the spinaxis. Sinusoidal electrical signals generated by the rate sensors 154,155 are coupled, respectively, via buffer amplifiers 160, 161 to asumming amplifier 162. Output signals generated by the accelerationsensor 156 are coupled via a buffer amplifier 164 and a 90° phase shiftcircuit 165 to a summing amplifier 166; output signals generated by theacceleration sensor 157 are coupled via a buffer amplifier 167 to thesumming amplifier 166. A rate G-sense nulling circuit 170 provides meansfor coupling selectable portions of the signals generated by theacceleration sensors 156, 157 to the summing amplifier 162 forcancelling undesired signals representative of cross-axis accelerationssensed by the rate sensors 154, 155. The output signals of the summingamplifiers 162, 166 drive, respectively, two linear voltage-to-frequencyconverters 172, 173 which generate a frequency-modulated pulse train andserve to transmit the FM signals off the rotating assembly 141 viaair-core transformers 174, 175. The frequency-modulated pulses areconverted back to analog voltages in frequency-to-voltage converters176, 177. The regenerated signals output from the frequency-to-voltageconverters 176, 177 are sinusoidal at the spin frequency of the rotatingassembly 141, having amplitude and phase representative of therespective rate and acceleration components sensed by the rotatingcrystal assemblies. Sin/Cos demodulators 180, 181 regenerate the analogvoltages representative, respectively, of angular rate about two axes,and linear acceleration along two axes. Timing signals from the timingcircuits 148 control the regeneration of the analog voltages; thesynchronizing signal from the sync pulse detector 151 allows adjustmentof the phase of the demodulator sampling function to compensate forphase shifts in the system. The demodulator 180, 181 output signals arefiltered to remove the carrier (spin frequency) harmonics, and arecoupled to an external user device such as an aircraft attitude andheading reference system via buffer amplifiers 186, 187, 188, 189.

Referring now to FIG. 7, a detailed electrical schematic diagram of therotating assembly 141 of FIG. 4 is shown. Reference characters of likecircuit elements are the same in FIGS. 4 and 7. The circuits depicted inFIG. 7 are divided generally into two groups; the circuits shown in theupper portion of FIG. 7 (the longer dimension being orientedhorizontally) and designated generally by reference character 54' arerate sensing circuits, the components of which are carried on theannular printed circuit board 54 shown in FIGS. 1 and 2. The circuitsdepicted in the lower portion of FIG. 7 and designated generally byreference character 56' are acceleration circuits, the components ofwhich are carried on the annular printed circuit board 56 shown in FIGS.1 and 2. The bender elements 154, 155, 156, 157 of FIG. 7 correspondwith the crystal assemblies 36, 38, 50, 52 of FIG. 1. The rate sensingpiezoelectric bender elements 154, 155 are connected, respectively, tonon-inverting inputs of the buffer amplifiers 160, 161. The rate sensors154, 155 are mounted diametrically opposed on the rotating assembly 141,and therefore are electrically connected to the respective amplifiers160, 161 in opposite electrical polarity with respect to each other asindicated by the literal designations A and B adjacent to each sensor154, 155 in FIG. 7, such that respective output signals at junctionpoints 190, 192 are in phase or additive. A balance potentiometer 194 isutilized during manufacture of the sensor assembly to compensate for anydifferences in the amplitude of the output signals at terminal points190, 192; such differences in signal amplitude may be caused by slightlydifferent circuit gains in the amplifiers 160, 161 or by imperfect,i.e., unmatched construction of the piezoelectric crystal assemblies154, 155.

The acceleration-sensing piezoelectric bender elements 156, 157 areconnected, respectively, to non-inverting inputs of the bufferamplifiers 164, 167. The output signal of the buffer amplifier 164 atjunction point 200 is coupled to the 90° phase shifter circuit 165. Thesignal output of the phase shift circuit 165 at junction point 202 iscoupled via a balance potentiometer 204 to the input of the summingamplifier 166 at junction point 206. The output signal of the bufferamplifier 167 at junction point 208 is coupled via the balancepotentiometer 204 to the input 206 of the summing amplifier 166. Thepiezoelectric bender elements 156, 157 are mounted on the rotatingassembly 141 angularly displaced from each other by 90°; the circuit 165shifts the phase of the signal generated by the bender element 156 by90° so that the respective signals at the junction points 202, 208 arein phase and additive when applied to the input 206 of the summingamplifier 166. Undesirable acceleration components sensed by theacceleration sensors 156, 157 are thereby cancelled in the summingamplifier 166 are previously described with reference to FIG. 5.

Cross-axis accelerations sensed by the rate sensors 154, 155 arecompensated for during manufacture of the sensor assembly by signalselection and adjustment in the nulling circuits 170. A portion of thesignal at junction point 200 representative of the acceleration sensedby the piezoelectric bender element 156 is coupled via an interboardjumper 212 to one of the input junction points 196, 214 of the summingamplifier 162, the amplitude of the signal being selected by adjustmentof a potentiometer 216 and the direction or polarity of the vector beingselected by installing an appropriate one of a pair of jumper wires orstraps 218. Similarly, a portion of the output signal at junction point208 representative of linear acceleration sensed by the piezoelectricbender element 157 is coupled via another interboard jumper wire 212 toone or the other of the input junction points 196, 214 of the summingamplifier 162, the amplitude of the coupled signal being selected byadjustment of a potentiometer 220, and the signal polarity beingselected by installation of an appropriate one of the jumper wires 218to connect the coupled signal either to the inverting 196 or thenon-inverting 214 input of the summing amplifier 162. The output signalof the summing amplifier 162 at junction point 222 consequently isrepresentative only of the desired angular-rates sensed by thepiezoelectric bender elements 154, 155, the cross-axis accelerationcomponents having been nulled out by compensating signals formed in thenulling circuits 170.

The rate signals at the junction point 222 are coupled through alinearizing circuit 224 to the voltage-to-frequency converter 172. Theoutput signal of the voltage-to-frequency converter 172 comprises aseries of pulses 4 microseconds wide having a center frequency ofapproximately 60 KHz which varies in frequency proportional to theapplied input voltage. The frequency-modulated output pulse train iscoupled to the transformer 174 for transmission off the rotating element141. The linearizing circuit 224 utilizes feedback from thevoltage-to-frequency converter 172 to a node at input junction point225. The linearizing circuit 224 serves to improve the linearity of therate signal, the rate signal having an inherently low level comparedwith the acceleration signal in the prevailing operating environment,i.e., during straight and level flight, when the vehicle experiences arelatively constant one-g acceleration field and comparatively very lowrate-signal perturbation. The output signal of the summing amplifier 166at junction point 210 is coupled to the voltage-to-frequency converter172. The output signal of the voltage-to-frequency converter 172 is aseries of pulses four microseconds wide having a center frequency offsetapproximately 2 KHz from the center frequency of thevoltage-to-frequency converter 172, which center frequency varies inproportion to the applied input voltage. The output signal of thevoltage-to-frequency converter 173 is coupled to the transformer 175 fortransmission off the rotating assembly 141. The center frequencies ofthe voltage-to-frequency converters 172, 173 are offset to precludecross-coupling of the acceleration signals to the rate signal circuits54' when the aircraft is in straight and level flight and the ratecircuits exhibit virtually zero signal output.

The circuits of FIG. 7 utilize commercially available integrated circuitcomponents. The summing amplifiers 160, 161, 164, 167 are LM108Aoperational amplifiers; the summing amplifiers 162, 166, the phase shiftcircuit 165 and the linearizing circuit 224 are LM1558 operationalamplifiers; the voltage-to-frequency converters 172, 173 are LM131Aintegrated circuits; all are manufactured by National SemiconductorCorporation. The power supply 146 on the rotating assembly 141 comprisesa full wave rectifier utilizing 1N4454 diodes, a positive regulator 226and a negative regulator 227. The regulators 226, 227 of thepresently-described embodiment are, respectively, μA78MO5 and LM120integrated circuit modules manufactured, respectively, by FairchildCamera and Instrument Corporation and National SemiconductorCorporation. Unregulated DC is supplied via interboard jumper wires 228to another set of regulators (not shown) on the acceleration circuitboard 56.

While the principles of the invention have been made clear in anillustrative embodiment, there will be immediately obvious to thoseskilled in the art many modifications of structure, arrangement,proportions, the elements, material and components that may be used inthe practice of the invention which are particularly adapted forspecific environments without departing from those principles. Theappended claims are intended to cover and embrace any such modificationswithin the scopes only of the true spirit and scope of the invention.

What is claimed is:
 1. A sensor assembly, comprising:a hysteresis motorhaving a central shaft fixed with respect to a major axis of referenceof a vehicle, a stator affixed to the shaft, and a rotating elementincluding a cylindrical housing, means for journaling the cylindricalhousing to the shaft, the cylindrical housing having a spin axis coaxialwith the shaft, and means mounted centrally disposed inside thecylindrical housing for driving the cylindrical housing, the drivingmeans being responsive to alternating current applied to the stator, thedriving means surrounding the stator and juxtaposed therewith, thecylindrical housing being driven at a substantially constant spinfrequency; a first cantilevered piezoelectric bender element mountedexteriorly on the cylindrical housing for rotation therewith, said firstbender element having a bending axis essentially normal to the spin axisof the cylindrical housing and an axis of sensitivity essentiallyparallel with the spin axis of the cylindrical housing; a secondcantilevered piezoelectric bender element mounted exteriorly on thecylindrical housing for rotation therewith, said second bender elementhaving a bending axis essentially parallel with the spin axis of thecylindrical housing and an axis of sensitivity essentially normal to thespin axis of the cylindrical housing; means mounted exteriorly on thecylindrical housing for bearing circuit means; first circuit means onsaid circuit-bearing means responsive to electrical signals generated bysaid first cantilevered piezoelectric bender element for generating afirst output signal at the spin frequency representative of angulardisplacement with respect to inertial space of said major axis ofreference of said vehicle; second circuit means on said circuit-bearingmeans responsive to electrical signals generated by said secondcantilevered piezoelectric bender element for generating a second outputsignal at the spin frequency representative of linear acceleration ofsaid vehicle in a plane perpendicular to said major axis of reference ofsaid vehicle; means for supplying power to said first and said secondcircuit means, said power supply means including a power transformerhaving primary winding affixed around the shaft and an annular secondarywinding coaxial with the primary winding, the secondary winding of thepower transformer being affixed to the cylindrical housing for rotationtherewith; and means for coupling the first and second output signalsoff the rotating element of said hysteresis motor, said coupling meansincluding a signal transformer having a secondary winding affixed to theshaft, and an annular primary winding coaxial with the secondarywinding, the annular primary winding being affixed to the cylindricalhousing for rotation therewith and coupled to at least one of said firstand said second circuit means, the secondary winding of the signaltransformer serving to couple the output signals of the at least one ofsaid first and said second circuit means to an external user device. 2.A sensor assembly as claimed in claim 1, wherein the journaling meanscomprises an end-cap bearing at either end of the cylindrical housing,the power transformer and said coupling means being inboard of theend-cap bearings and enclosed inside the cylindrical housing.
 3. Asensor assembly as claimed in claim 1, comprising:a cover; and saidcircuit-bearing means comprising a pair of annular printed-circuitboards each forming one of two side walls of a compartment enclosingsaid first and said second piezoelectric bender elements, the componentsof said first and said second circuit means, the cover extending betweenperipheral ends of the pair of annular printed-circuit boards.
 4. Asensor assembly as claimed in claims 1, 2, or 3, furthercomprising:means for generating a signal representative of a referenceposition of the rotating element of said hysteresis motor.
 5. A sensorassembly, comprising:an inside-out motor having a central shaft fixedwith respect to a major axis of reference of a vehicle, a stator affixedto the shaft, and a rotating element including a motor housing, meansfor journaling the motor housing to the shaft, the motor housing havinga spin axis coaxial with the shaft, and a hysteresis ring mounted insidethe motor housing and juxtaposed with the stator, the hysteresis ringdriving the rotating element at a substantially constant spin frequencyin response to alternating current applied to the stator; a first pairof cantilevered piezoelectric bender elements mounted oppositelydisposed from each other exteriorly on the motor housing for rotationtherewith, each of said first pair of bender elements having a bendingaxis essentially normal to the spin axis of the motor housing and anaxis of sensitivity essentially parallel with the spin axis of the motorhousing; a second pair of cantilevered piezoelectric bender elementsmounted exteriorly on the motor housing for rotation therewith, saidsecond pair of bender elements having a bending axis essentiallyparallel with the spin axis of the motor housing and an axis ofsensitivity essentially normal to the spin axis of the motor housing,the axes of sensitivity of said second pair of bender elements beingangularly displaced from each other; a circuit-bearing substrate mountedexteriorly on the motor housing for rotation therewith; first circuitmeans on said circuit-bearing substrate responsive to electrical signalsgenerated by said first pair of cantilevered piezoelectric benderelements for generating a first output signal representative of angulardisplacement with respect to inertial space of said major axis ofreference of said vehicle; second circuit means on said circuit-bearingsubstrate responsive to electrical signals generated by said second pairof cantilevered piezoelectric bender elements for generating a secondoutput signal representative of linear acceleration of said vehicle in aplane perpendicular to said major axis of reference of said vehicle;means for supplying power to said first and said second circuit means,said power supply means including a power transformer having a primarywinding affixed around the shaft and an annular secondary windingconcentric with the primary winding, the annular secondary winding ofthe power transformer being affixed inside the motor housing forrotation therewith; and first and second means for coupling,respectively, the first and the second output signals off the rotatingelement of said inside-out motor, each said coupling means including asignal transformer having a secondary winding affixed around the shaftand an annular primary winding concentric with the secondary winding,the annular primary winding being affixed inside the motor housing forrotation therewith and coupled to a corresponding one of said first andsaid second circuit means, the secondary winding serving to couple theoutput signal of the corresponding one of said first and the secondcircuit means to an external user device.
 6. A sensor assembly asclaimed in claim 5, comprising:a cover; and said circuit-bearingsubstrate includes a pair of annular printed-circuit boards each formingone of two side walls of a compartment enclosing said first and saidsecond pairs of cantilevered piezoelectric bender elements andcomponents of said first and said second circuit means, the coverextending between peripheral ends of the pair of annular printed-circuitboards.
 7. A sensor assembly as claimed in claim 5, wherein thejournaling means comprises an end-cap baring at either end of the motorhousing enclosing the power transformer and said first and said secondcoupling means inside the cylindrical housing.